Selective growth method, and semiconductor light emitting device and fabrication method thereof

ABSTRACT

At the time of selective growth of an active layer on a substrate, crystal is previously grown in an active layer non-growth region, and the active layer is grown in an active layer selective growth region. With this configuration, a source supplied to the non-growth region is incorporated in the deposited crystal from the initial stage of growth, so that the supplied amount of the source to the active layer selective growth region is kept nearly at a constant value over the entire period of growth of the active layer, to eliminate degradation of characteristics of the device due to a variation in growth rate of the active layer. In particular, the selective growth method is effective in fabrication of a semiconductor light emitting device including a cladding layer, a guide layer, and an active layer, each of which is formed by selective growth, wherein the active layer has multiple quantum wells.

The subject matter of application Ser. No. 10/341,827 is incorporatedherein by reference. The present application is a divisional of U.S.application Ser. No. 10/341,827, filed Jan. 14, 2003 now U.S. Pat. No.6,969,670, which claims priority to Japanese Patent Application No.JP2002-009285, filed Jan. 17, 2002. The present application claimspriority to these previously filed applications.

BACKGROUND OF THE INVENTION

The present invention relates to a selective growth method, and to asemiconductor light emitting device and a fabrication method thereof, toeach of which the selective growth method is applied.

GaN based compound semiconductors have become a focus of attention assemiconductor materials for semiconductor light emitting devices, and avariety of device designs and trials have been made to improvecharacteristics of semiconductor light emitting devices using GaN basedcompound semiconductors.

The GaN based semiconductor light emitting device emits light having awavelength in a short-wavelength region, and therefore, it allowsemission of light of blue or green. Accordingly, a full-color imagedisplay unit can be fabricated, for example, by combining the GaN basedsemiconductor light emitting devices with GaAs based semiconductor lightemitting devices allowing emission of light of red.

The above-described GaN based semiconductor light emitting device can befabricated by forming a mask having an opening on a sapphire substrate,forming a nitride layer by selective growth from the opening, andsequentially forming a cladding layer, a guide layer, and an activelayer on a tilt growth plane of the nitride layer by selective growth.Such a light emitting device is excellent in luminous efficiency.

In the step of selective growth of the active layer in theabove-described fabrication method, it is required to grow the activelayer at a low temperature, and such growth of the active layer causes aproblem that the growth rate is reduced with an increase in thickness ofthe active layer, and the repeatability of fabrication is also degraded.For example, in the case of forming an active layer of a multi-layerstructure having multiple quantum wells, there occurs a phenomenon thatthe growth rate of the active layer becomes gradually low and therebythe thickness of the quantum well becomes gradually thin. In a lightemitting device including an active layer of the multi-quantum wellstructure, to ensure excellent light emission characteristics, it ispreferred that the thickness of one of the multiple quantum wells isequal to that of another of the multiple quantum wells. If the thicknessof the quantum well becomes gradually thin, characteristics of thedevice are degraded, and more specifically, a half-value width of anemission wavelength peak becomes large, and the repeatability offabrication is also degraded.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a selective growthmethod capable of solving a problem associated with degradation ofcharacteristics due to a variation in growth rate of an active layer.

Another object of the present invention is to provide a semiconductorlight emitting device and a fabrication method thereof, which arecapable of improving characteristics, for example, reducing a half-valuewidth of an emission wavelength peak.

The present inventors have examined to achieve the above objects, andfound the following knowledge.

In the case of forming an active layer at a low temperature by selectivecrystal growth, the amount of deposition on a mask as an active layernon-growth region is increased with an increase in thickness of theactive layer as the low temperature layer, and along the increasedamount of the deposition on the mask, the supplied amount of a source toa device growth region, that is, an active layer selective growth regionis reduced. As a result, if the growth of the active layer begins in astate with no deposition on the mask, the growth rate becomes graduallylow. As a result, the thickness of the active layer becomes graduallythin, and the repeatability of fabrication is also degraded.

The present invention has been accomplished on the basis of suchknowledge.

According to a first aspect of the present invention, there is provideda selective growth method for selectively growing an active layer on asubstrate, including the steps of previously growing crystal in anactive layer non-growth region, and growing an active layer in an activelayer selective growth region.

As described above, if deposition (crystal) is present in the non-growthregion (mask), the source supplied to the non-growth region isincorporated in the crystal, with a result that the supplied amount ofthe source to the active layer selective growth region is reduced.

In the case where the growth of the active layer begins in a state withno deposition in the non-growth region, in the initial stage, since allof the source is substantially supplied to the active layer selectivegrowth region, the supplied amount of the source to the active layerselective growth region is large; however, in the later stage, sincedeposition is formed in the non-growth region with elapsed time, thesource is partially incorporated in the deposition, with a result thatthe supplied amount of the source to the active layer selective growthregion is reduced. On the contrary, in the case where the growth of theactive layer begins in a state in which deposition is already formed inthe non-growth region, the source supplied to the non-growth region isincorporated in the deposition from the initial stage of growth, so thatthe supplied amount of the source to the active layer selective growthregion is kept nearly at a constant value. As a result, it is possibleto keep the growth rate of the active layer constant, and hence toeliminate a variation in thickness of the active layer.

According to a second aspect of the present invention, there is provideda semiconductor light emitting device including a cladding layer, aguide layer, and an active layer, each of which is formed by selectivegrowth, wherein the active layer has multiple quantum wells, and thethickness of one of the multiple quantum wells is nearly equal to thatof another of the multiple quantum wells.

According to a third aspect of the present invention, there is provideda method of fabricating a semiconductor light emitting device includinga cladding layer, a guide layer, and an active layer, each of which issequentially formed by selective growth, the method including the stepsof previously growing crystal in an active layer non-growth region, andgrowing the active layer in an active layer selective growth region.

According to the semiconductor light emitting device of the presentinvention, since the active layer formed by selective growth hasmultiple quantum wells and the thickness of one of the multiple quantumwells is nearly equal to that of another of the multiple quantum wells,it is possible to enhance characteristics of the device, for example,reduce a half-value width of an emission wavelength peak. Further,according to the method of fabricating a semiconductor light emittingdevice, it is possible to fabricate a semiconductor light emittingdevice having excellent characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following description taken inconnection with the accompanying drawings wherein:

FIGS. 1A and 1B are a schematic sectional view and a schematic plan viewshowing one example of a semiconductor light emitting device,respectively;

FIGS. 2A to 2E are schematic sectional views showing one example of aprocess of fabricating a semiconductor light emitting device accordingto the present invention, wherein FIG. 2A shows a step of forming anunderlying growth layer, FIG. 2B shows a step of forming a mask layer,FIG. 2C shows a step of forming a crystal growth layer by selectivegrowth, FIG. 2D shows a step of growing crystal on a mask layer, andFIG. 2E shows a step of forming an active layer and a p-type GaN layerby selective growth;

FIGS. 3A and 3B are typical views showing the supplied state of a sourcegas at the time of selective growth of an active layer, wherein FIG. 3Ashows the supplied state of the source gas in the case where any crystalhas not been grown on a mask, and FIG. 3B shows the supplied state ofthe source gas in the case where crystal has been grown on the mask;

FIG. 4 is a timing chart with which a source gas is supplied inaccordance with a related art method;

FIG. 5 is a timing chart with which a source gas is supplied inaccordance with a selective growth method of the present invention;

FIG. 6 is a schematic perspective view showing a first shape example ofa semiconductor light emitting device of the present invention;

FIG. 7 is a schematic perspective view showing a second shape example ofthe semiconductor light emitting device of the present invention;

FIG. 8 is a schematic perspective view showing a third shape example ofthe semiconductor light emitting device of the present invention;

FIG. 9 is a schematic perspective view showing a fourth shape example ofthe semiconductor light emitting device of the present invention; and

FIG. 10 is a schematic perspective view showing a fifth shape example ofthe semiconductor light emitting device of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a selective growth method, and a semiconductor lightemitting device and a fabrication method thereof, to each of which thepresent invention is applied, will be described in detail with referenceto the drawings.

The selective growth of an active layer according to the presentinvention will be described by example of a process of fabricating asemiconductor light emitting device having a structure shown in FIGS. 1Aand 1B.

FIGS. 1A and 1B are a sectional view and a plan view showing thestructure of a semiconductor light emitting device as one example of adevice to which the fabrication process of the present invention isapplied, respectively.

The light emitting device shown in the figures is exemplified by a GaNbased light emitting diode, which is formed by crystal growth on, forexample, a sapphire substrate. Such a GaN based light emitting diodeformed on the sapphire substrate has a feature that it can be easilypeeled from the sapphire substrate by laser irradiation. Morespecifically, when an interface between the sapphire substrate and a GaNbased growth layer of the GaN based light emitting diode is irradiatedwith laser beams passing through the sapphire substrate, laser abrasionoccurs at the interface, to cause film peeling at the interface by aphenomenon that nitrogen (N) of GaN is vaporized.

The GaN based light emitting diode shown in the figures has a structurethat a hexagonal pyramid shaped GaN layer 2 is formed by selectivecrystal growth on an underlying growth layer 1 made from a GaN basedsemiconductor. While not shown, an insulating film having an opening isformed as a mask on the underlying growth layer 1, and the hexagonalpyramid shaped GaN layer 2 is formed by selective crystal growth fromthe opening of the insulating film by an MOCVD process or the like. Ifthe C-plane of sapphire is used as the principle plane of the sapphiresubstrate for forming the GaN layer 2 thereon by crystal growth, the GaNlayer 2 becomes a growth layer having a pyramid shape covered with anS-plane, that is, (1-101) plane. The GaN layer 2 is a region doped withsilicon. The tilt S-plane portion of the GaN layer 2 functions as acladding portion of a double-hetero structure.

An active layer 3 made from InGaN is formed so as to cover the tiltS-plane of the GaN layer 2. The active layer 3 may be a single layer, ora layer having a multi-quantum well (MQW) structure.

The fabrication process according to the present invention can beadvantageously applied to the GaN based light emitting diode, if thediode includes the active layer 3 having the MQW structure.

The active layer 3 having the MQW structure may be formed, for example,by repeatedly stacking units each having an InGaN layer as a quantumwell and an GaN layer containing no indium (In) as a barrier. Thethickness of each of the quantum well and the barrier may be suitablyselected (in general, to about several nm), and the number of times ofrepeated stacking may be suitably selected.

A GaN layer 4 doped with magnesium is formed on the outer surface of theactive layer 3. The GaN layer 4 doped with magnesium also functions as acladding portion.

The light emitting diode has a p-electrode 5 and an n-electrode 6. Ametal material such as Ni/Pt/Au or Ni(Pd)/Pt/Au is vapor-deposited onthe GaN layer 4 doped with magnesium, to form the p-electrode 5. A metalmaterial such as Ti/Al/Pt/Au is vapor-deposited in an opening formed inthe above-described insulating film (not shown), to form the n-electrode6. In the case of extracting an n-electrode from the back surface sideof the underlying growth layer 1, it is not required to form then-electrode 6 on the front surface side of the underlying growth layer1.

The process of fabricating the above-described semiconductor lightemitting device will be described below.

As shown in FIG. 2A, an n-type GaN layer 11 is formed as an underlyinggrowth layer on a growth substrate 10 such as a sapphire substrate by,for example, an MOCVD process. The n-type GaN layer 11 is not requiredto be n-type conductive as a whole, but may be n-type conductive only atits uppermost surface. As one example, the n-type GaN layer 11 can beformed by doping silicon in a GaN layer.

As shown in FIG. 2B, a mask layer 12 as an anti-growth film made fromsilicon oxide, silicon nitride, or tungsten is formed overall on then-type GaN layer 11 by, for example, a CVD process, and a plurality ofhexagonal window regions 13 are formed at positions, corresponding tothose of device formation regions, of the mask layer 12.

As shown in FIG. 2C, an n-type GaN layer 14 as a crystal growth layer isformed by selective crystal growth from each of the window regions 13.The n-type GaN layer 14 formed into an approximately hexagonal pyramidshape functions as a cladding layer. The tilt side plane of the n-typeGaN layer 14 is an S-plane.

An active layer made from InGaN and a p-type GaN layer are sequentiallystacked on the tilt side plane of the n-type GaN layer 14.

According to the fabrication process of the present invention, however,prior to the formation of the active layer, as shown in FIG. 2D, crystal15 is grown in the polycrystalline form on a portion, on which then-type GaN layer 14 is not formed by crystal growth, of the surface ofthe mask layer 12.

The growth of the crystal 15 may be performed as follows: namely, thesame source gas as that used for the GaN layer 14 is used, and thecondition of crystal growth of GaN for forming the GaN layer 14 ischanged to allow crystal growth of GaN on the mask layer 12. Thecondition to allow crystal growth of GaN on the mask layer 12 may be setsuch that the carrier gas is changed into nitrogen gas at the time ofcrystal growth of GaN by the MOCVD process, or the growth temperature islowered at the time of crystal growth of GaN by the MOCVD process.

After the crystal 15 is thus grown on the mask layer 12, as shown inFIG. 2E, an active layer 16 made from InGaN and a p-type GaN layer 17are sequentially formed by crystal growth on the S-plane of the n-typeGaN layer 14. At this time, the active layer 16 extends along the tiltS-plane of the GaN layer 14 as the crystal growth layer, and therefore,it is not in parallel to the principal plane of the growth substrate 10but is tilted therefrom. As a result, the area of the active layer 16becomes sufficiently larger than each of the area of the window region13 and the projected area of the GaN layer 14 as the crystal growthlayer.

In the above-described fabrication process, it is important to grow thecrystal 15 on the mask layer 12 prior to crystal growth of the activelayer 16. The previous growth of the crystal 15 on the mask layer 12makes it possible to eliminate a difference in film thickness betweenthe initial stage and the later stage of crystal growth, morespecifically, a difference between the thickness of a quantum well (orbarrier) of the active layer 16 formed in the initial stage and thethickness of a quantum well (or barrier) of the active layer 16 formedin the later stage.

FIGS. 3A and 3B typically show how the supply state of a source gasdiffers depending on the presence or absence of the crystal 15.

If any crystal 15 is not formed on the mask layer 12, in the initialstage of crystal growth of the active layer 16, as shown in FIG. 3A, asource gas having been supplied to the mask layer 12 as the non-growthregion flows in the direction shown by arrows, to contribute to crystalgrowth of the active layer 16 on the S-plane of the GaN layer 14.Accordingly, in the initial stage of crystal growth of the active layer16, the amount of the source gas contributing to crystal growth of theactive layer 16 becomes large. Meanwhile, as the crystal growth of theactive layer 16 proceeds, crystal 15 begins to grow on the mask layer12. Consequently, as shown by arrows in FIG. 3B, the source gas havingbeen supplied to the mask layer 12 as the non-growth region is used forcrystal growth of the crystal 15 or adsorbed on the crystal 15, andthereby not supplied to the S-plane of the GaN layer 14. Accordingly, inthe later stage of crystal growth of the active layer 16, the amount ofthe source gas contributing to crystal growth of the active layer 16becomes small. Such a difference in supplied amount of the source gasbetween the initial stage and the later stage of crystal growth leads toa difference between the thickness of a quantum well (or barrier) of theactive layer 16 in the initial stage and the thickness of a quantum well(or barrier) of the active layer 16 in the later stage.

On the contrary, according to the fabrication process of the presentinvention, since the crystal 15 is grown on the mask layer 12 prior tocrystal growth of the active layer 16, the supplied state of the sourcegas can be kept as that shown in FIG. 3B throughout the entire periodfrom the initial stage to the later stage of crystal growth. As aresult, the supplied amount of the source gas to the S-plane of the GaNlayer 14 can be kept nearly constant, to eliminate the difference inthickness of the active layer 16 between the initial stage and the laterstage of crystal growth.

The present inventors have made experiments to prove the above-describedeffect. In the experiments, the active layer 16 having a multi-quantumwell (MQW) structure was formed by alternately stacking five quantumwells (InGaN) and five barriers (GaN), and the thickness distribution ofthe quantum wells and the barriers of the active layer 16 was measured.

Table 1 shows the thickness distribution of quantum wells and barriersof the active layer 16 formed under the condition that any crystal 15 isnot grown on the mask layer 12 prior to formation of the active layer 16according to the related-art method.

In formation of the active layer 16, the source gas [trimethyl Ga (TMGa)and trimethyl In (TMIn)] was supplied with a timing shown in FIG. 4, andeach quantum well (QW) was formed by crystal growth at 750° C.

In this experiment, as shown in Table 1, the thickness of each of thequantum well and the barrier is gradually reduced in the later stage.For example, there is a difference of 0.5 nm between the thickness ofthe first quantum well and the thickness of the fifth quantum well.

TABLE 1 1^(st) 2^(nd) 3^(rd) 4^(th) 5^(th) Quantum 1^(st) Quantum 2^(nd)Quantum 3^(rd) Quantum 4^(th) Quantum 5^(th) Well Barrier Well BarrierWell Barrier Well Barrier Well Barrier Film 3.0 5.0 2.7 4.8 2.6 4.5 2.54.3 2.5 4.3 Thickness (nm)

Table 2 shows the thickness distribution of quantum wells and barriersof the active layer 16 formed under the condition that the crystal 15 isgrown on the mask layer 12 prior to formation of the active layer 16according to the fabrication method of the present invention.

In formation of the active layer 16, the source gas [trimethyl Ga(TMGa)and trimethyl In (TMIn)] was supplied with a timing shown in FIG. 5, andthe crystal 15 was grown on the mask layer 12 in the initial period of400 seconds. Like the previous experiment, each quantum well (QW) wasformed by crystal growth at 750° C.

In this experiment, as shown in Table 2, the thickness of each of thequantum well and the barrier is little reduced even in the later stage.As a result, it becomes apparent that the reduction in thickness of theactive layer 16 in the later stage of crystal growth can be certainlysuppressed.

TABLE 2 1^(st) 2^(nd) 3^(rd) 4^(th) 5^(th) Quantum 1^(st) Quantum 2^(nd)Quantum 3^(rd) Quantum 4^(th) Quantum 5^(th) Well Barrier Well BarrierWell Barrier Well Barrier Well Barrier Film 2.5 4.2 2.4 4.2 2.5 4.1 2.44.0 2.4 4.1 Thickness (nm)

The present invention will be hereinafter more fully described byexample of the following semiconductor light emitting device, whereinthe selective growth method of the present invention is applied tocrystal growth of an active layer of the semiconductor light emittingdevice.

It is to be noted that the following semiconductor light emitting deviceis for illustrative purposes only, and therefore, the present inventionis not limited thereto.

The semiconductor light emitting device to which the present inventionis applied is represented by a semiconductor light emitting devicefabricated by forming, on a substrate, a crystal layer having a tiltcrystal plane (for example, S-plane) tilted from the principal plane ofthe substrate, and sequentially forming a first conductive type layer,an active layer, and a second conductive type layer in such a mannerthat each of these layers extends within a plane parallel to the tiltcrystal plane of the crystal layer.

The substrate used herein is not particularly limited insofar as itallows a crystal layer having a tilt crystal plane tilted from theprincipal plane of the substrate to be formed thereon, and may beselected from various substrates, for examples, substrates made fromsapphire (Al₂O₃, having A-plane, R-plane, or C-plane), SiC (including6H, 4H, and 3C), GaN, Si, ZnS, ZnO, AlN, LiMgO, GaAs, MgAl₂O₄, andInAlGaN. Of these substrates, hexagonal or cubic crystal basedsubstrates are preferred, with the hexagonal substrates being mostpreferred.

In the case of using a sapphire substrate, the C-plane of sapphire maybe taken as the principal plane of the substrate. In general, thesapphire substrate with the C-plane of sapphire taken as the principalplane thereof has been often used to grow a gallium nitride (GaN) basedcompound semiconductor thereon. It is to be noted that the C-plane ofsapphire taken as the principal plane of the sapphire substrate is notlimited to the theoretical C-plane but may be a plane tilted from thetheoretical C-plane by an angle 5 to 6 degrees.

The substrate may not be a constituent of a light-emitting device as aproduct. In other words, the substrate may be used merely to hold adevice portion and be removed before the device is accomplished.

The crystal layer formed on the substrate has a tilt crystal planetilted from the principal plane of the substrate. The crystal layer isnot particularly limited insofar as it allows a light-emitting region(to be described later) composed of a first conductive type layer, anactive layer, and a second conductive type layer to be form on a planeparallel to the tilted crystal plane, tilted from the principal plane ofthe substrate, of the crystal layer. In general, the crystal layer ispreferably made from a material having a wurtzite type crystalstructure.

For example, such a crystal layer may be made from a material selectedfrom a group III based compound semiconductor, a BeMgZnCdS basedcompound semiconductor, a BeMgZnCdO based compound semiconductor, agallium nitride (GaN) based compound semiconductor, an aluminum nitride(AlN) based compound semiconductor, an indium nitride (InN) basedcompound semiconductor, an indium gallium nitride (InGaN) based compoundsemiconductor, and an aluminum gallium nitride (AlGaN) based compoundsemiconductor. Of these materials, a nitride semiconductor such as agallium nitride based compound semiconductor is preferably used as thematerial for forming the crystal layer.

It is to be noted that according to the present invention, the nitridesemiconductor expressed by InGaN, AlGaN, or GaN does not necessarilymean only InGaN, AlGaN, or GaN in the form of a strict ternary or binarymixed crystal. For example, the nitride semiconductor expressed by InGaNmay contain a trace amount of Al and other impurities which do notaffect the function of InGaN without departing from the scope of thepresent invention.

The crystal layer can be formed by a chemical vapor deposition processselected, for example, from a metal organic chemical vapor deposition(MOCVD) process including a metal organic vapor phase epitaxy (MOVPE)process, a molecular beam epitaxy (MBE) process, and a hydride vaporphase epitaxy (HVPE) process. In particular, the MOCVD process ispreferred because it rapidly yields a crystal layer with a desirablecrystallinity. The MOCVD method commonly employs alkyl metal compounds,such as TMG (trimethylgallium) or TEG (triethylgallium) as a Ga source,TMA (trimethylaluminum) or TEA (triethylaluminum) as an Al source, andTMI (trimethylindium) or TEI (triethylindium) as an In source. It alsoemploys ammonia gas or hydrazine gas as a nitrogen source, and othergases as an impurity source, for example, silane gas for Si, germane gasfor Ge, Cp₂Mg (cyclopentadienylmagnesium) for Mg, and DEZ (diethylzinc)for Zn. In the general MOCVD process, the gases are fed to the surfaceof the substrate heated at about 600° C. or more, and are decomposed toform a layer of an InAlGaN based compound semiconductor by epitaxialgrowth.

It is preferred to form an underlying growth layer on the substrate andto form the crystal layer on the underlying growth layer.

The underlying growth layer can be formed by the same chemical vapordeposition process as that used for forming the crystal layer, forexample, the metal organic chemical vapor deposition (MOCVD) process,molecular beam epitaxy (MBE) process, or hydride vapor phase epitaxy(HVPE) process.

The underlying growth layer may be made from, for example, galliumnitride or aluminum nitride, and may have a structure composed of acombination of a low-temperature buffer layer and a high-temperaturebuffer layer, or a combination of a buffer layer and a crystal seedlayer functioning as a crystal seed.

The above structure of the underlying growth layer will be described indetail below.

If the crystal layer is formed by crystal growth from a low-temperaturebuffer layer, there occurs a problem that polycrystals tend to bedeposited on the mask layer. To solve such a problem, a high-temperaturebuffer layer may be formed on the low-temperature buffer layer and thenthe crystal layer be formed thereon so as to be grown along a planedifferent from the principal plane of the substrate. With thisconfiguration, the crystal layer with a desirable crystallinity can beformed by crystal growth.

In the case of using no crystal seed layer at the time of forming thecrystal layer, the crystal layer is required to be formed by selectivecrystal growth from a buffer layer. At this time, however, there occursa problem that crystal growth is liable to occur even in an anti-growthregion where the crystal growth is not required. To solve such aproblem, a crystal seed layer may be formed on the buffer layer and thecrystal layer be formed by selective crystal growth from the crystalseed layer. With this configuration, the crystal layer can beselectively formed in a region where the crystal growth is required.

The low-temperature buffer layer is intended to relieve lattice mismatchbetween the substrate and a nitride semiconductor. Accordingly, if thesubstrate has a lattice constant close to or identical to that of anitride semiconductor, the low-temperature buffer layer is notnecessarily provided. For example, an AlN layer may be grown on an SiCsubstrate as a high-temperature buffer layer without lowering the growthtemperature, and an AlN or GaN layer may be grown on an Si substrate asa high-temperature buffer layer without lowering the growth temperature.Even in this case, a GaN layer with a desirable crystallinity can beformed by crystal growth on the buffer layer. Additionally, in the caseof using a GaN substrate, the structure without any buffer layer may beadopted.

In fabrication of the semiconductor light emitting device according tothis embodiment, the crystal layer having a tilt crystal plane tiltedfrom the principal plane of the substrate is formed by using theselective growth process.

The tilt crystal plane, tilted from the principal plane of thesubstrate, of the crystal layer is grown depending on the kind of theprincipal plane of the substrate.

If the crystal layer is grown on the (0001) plane [C-plane] as theprincipal plane of the substrate having the wurtzite type crystalstructure, the tilt crystal plane of the crystal layer becomes oneselected from the (1-100) plane [M-plane], the (1-101) plane [S-plane],the (11-20) plane [A-plane], the (1-102) plane [R-plane], the (1-123)plane [N-plane], the (11-22) plane, and crystal planes equivalentthereto. In particular, it is preferred to grow the crystal layer withthe S-plane or the (11-22) plane, or the crystal plane equivalentthereto. It is to be noted that the crystal plane equivalent to theS-plane or the (11-22) plane is the crystal plane tilted from theS-plane or the (11-22) plane by an angle of 5 to 6 degrees.

In particular, the S-plane is a stable plane selectively grown on theC⁺-plane and is therefore relatively obtainable. The S-plane isexpressed by the (1-101) plane in accordance with Miller indices of ahexagonal crystal system. Just as the C-plane includes the C⁺-plane andthe C⁻-plane, the S-plane includes the S⁺-plane and the S⁻-plane. Inthis specification, the S⁺-plane is grown on the C⁺-plane of GaN, and itis referred to as the S-plane unless otherwise stated. Of the S-planes,the S⁺-plane is stable. In addition, the Miller index of the C⁺-plane is(0001).

In the case of growing the S-plane of the crystal layer made from agallium nitride based compound semiconductor on the C⁺-plane of thesubstrate as described above, the number of bonds from Ga to N on theS-plane is 2 or 3, which number is second to that on the C-plane. Sincethe C⁻-plane cannot be grown on the C⁺-plane in practice, the number ofbonds on the S-plane is the largest.

In the case of growing a wurtzite type nitride, for example, GaN basednitride on a sapphire substrate with the C-plane of sapphire taken asthe principal plane thereof, if the selective growth process is not usedto grow the nitride, the surface of the nitride is grown as theC⁺-plane, whereas if the selective growth process is used to grow thenitride, the surface of the nitride can be grown as the S-plane tiltedfrom the C-plane of the sapphire substrate.

On the C⁺-plane, parallel to the C-plane of the substrate, of thenitride, the bond of N liable to be easily released from the planecombines with one bond of Ga, whereas on the S-plane, tilted from theC-plane of the substrate, of the nitride, the bond of N combines with atleast one bond of Ga.

As a result, the V/III ratio of the nitride grown along the S-plane canbe effectively increased, to advantageously improve the crystallinity ofthe laminated structure. In addition, according to the formation of thenitride by the selective growth process, since nitride is grown alongthe S-plane different from the orientation of the substrate,dislocations extending upwardly from the substrate may be bent, toadvantageously reduce crystal defects of the nitride.

In the semiconductor light emitting device according to this embodiment,as described above, the crystal layer has a tilt crystal plane tiltedfrom the principal plane of the substrate.

The structure of the crystal layer will be more fully described below.

The crystal layer may have an approximately hexagonal pyramid shape inwhich the tilt plane forming the pyramid shape is composed of theS-plane or a plane substantially equivalent thereto. Alternatively, thecrystal layer may have a so-called approximately hexagonal truncatedpyramid shape in which the tilt plane of the truncated pyramid shape iscomposed of the S-plane or a plane substantially equivalent thereto, andthe upper flat plane of the truncated pyramid shape is composed of theC-plane or a plane substantially equivalent thereto.

Each of the approximately hexagonal pyramid shape and the approximatelyhexagonal truncated pyramid shape is not necessarily a perfect hexagonalshape but may be an imperfect hexagonal shape with one or more missingfaces.

In a preferred embodiment, the tilt crystal plane is hexagonal and isarranged so as to be approximately symmetrical. The term “approximatelysymmetrical” used herein embraces not only completely symmetrical butalso slightly asymmetrical.

The ridge between adjacent two crystal plane segments of the crystallayer is not necessarily a straight line. Also, each of theapproximately hexagonal pyramid shape and the approximately hexagonaltruncated pyramid shape may extend in straight line.

The concrete selective growth process used for selectively growing thecrystal layer will be described below.

The selective growth of the crystal layer is performed by making use ofa selectively removed portion of the underlying growth layer, or bymaking use of a selectively formed opening in a mask layer which isformed on or under the underlying growth layer.

For example, if the underlying growth layer is composed of a bufferlayer and a crystal seed layer, the crystal seed layer is formed on thebuffer layer in such a manner as to be divided into scattered smallregions each having a diameter of about 10 μm, and the crystal layerhaving the S-plane or the like is formed by crystal growth from each ofthe small regions. For example, the divided regions of the crystal seedlayer may be arranged so as to be spaced from each other at intervals ofa value equivalent of a margin for separation of adjacent light emittingdevices. The divided small region may be formed into a shape selectedfrom a stripe, a lattice, a circle, a square, a hexagon, a triangle, arectangle, a rhombus, and other shapes deformed therefrom.

The selective growth of the crystal layer may be performed by forming amask layer on the underlying growth layer, and selectively formingwindow regions in the mask layer. The mask layer may be made fromsilicon oxide or silicon nitride. The crystal layer having anapproximately hexagonal truncated pyramid shape or an approximatelyhexagonal pyramid shape extending in straight line in one longitudinaldirection as described above can be formed by selective crystal growthfrom each of stripe-shaped window regions formed in the mask layer orfrom each of stripe-shaped regions of the crystal seed layer.

By forming, in the mask layer, the window region of a circular shape (ora hexagonal shape whose one side extends along the (1-100) direction or(11-20) direction) having a size of around 10 μm, it is possible toeasily form the crystal layer having a size of about twice as large asthe window region by selective growth from the window region. In thecrystal layer thus formed by selective growth, since the S-plane tiltedfrom the principal plane of the substrate has an effect of bending andblocking dislocations extending from the substrate, it is possible toreduce the density of dislocations in the crystal layer.

The present inventors have made an experiment to examine characteristicsof the S-plane of a semiconductor light emitting device.

A semiconductor light emitting device was prepared by forming a crystallayer of a hexagonal truncated pyramid shape having the S-plane byselective growth, and sequentially growing an InGaN active layer and aMg-doped layer on the S-plane of the crystal layer.

With respect to such a semiconductor light emitting device, the state ofeach layer grown along the S-plane was examined.

As a result of observation of the state of the S-plane by making use ofcathode luminescence, it was revealed that the crystallinity of theS-plane is desirable, and therefore, the luminous efficiency on theS-plane is higher than that on the C⁺-plane.

In particular, since the growth temperature of the InGaN active layer isin a range of 700 to 800° C., the decomposition efficiency of ammonia islow, with a result that the growth of the InGaN active layer requires alarger amount of nitrogen species. In this regard, the growth of theInGaN active layer on the S-plane is preferred. As a result ofobservation of the surface state of the S-plane by AFM (Atomic ForceMicroscopy), it was revealed that the surface state of the S-plane is aregular stepped state suitable for growth of InGaN thereon.

As a result of observation by AFM, it was also revealed that althoughthe state of the growth surface of the Mg-doped layer is generally poorin the level observed by AFM, the Mg-doped layer can be grown along theS-plane while keeping a desirable surface state, and that the dopingcondition at the time of growth on the S-plane is quite different fromthat at the time of growth on a plane other than the S-plane.

The S-plane was further subjected to microscopic photoluminescencemapping having a resolving power of about 0.5 to 1 μm. The result showedthat although the surface of the sample grown on the C⁺-plane by theordinary growth process has irregularities at a pitch of about 1 μm, thesurface of the sample grown on the S-plane is uniform.

In addition, as a result of observation of SEM (scanning electronmicroscope), it was revealed that the flatness of the tilt plane of thelayer grown on the S-plane obtained by the selective growth process issmoother than the flat plane of the layer grown along the C⁺-planeobtained by the ordinary growth process.

In the case of forming a crystal layer by selective growth from a windowregion formed in a selective growth mask, the crystal layer is generallygrown only in an area over the window region. In this case, to realizelateral growth of the crystal layer, there may be adopted amicro-channel epitaxy process. The use of the micro-channel epitaxyprocess allows the crystal layer to be laterally grown into a shapelarger than the window region.

It is known that the lateral growth of the crystal growth by using themicro-channel epitaxy process is effective to prevent threadingdislocations extending from the substrate from being propagated in thecrystal layer and hence to reduce the density of dislocations in thecrystal layer. The lateral growth of the crystal layer by using themicro-channel epitaxy process is also advantageous in increasing thelight-emitting region, equalizing a current, avoiding concentration ofcurrent, and reducing the current density.

In the semiconductor light emitting device according to this embodiment,as described above, a crystal layer having a tilt crystal plane tiltedfrom the principal plane of a substrate is formed, and a firstconductive type layer, an active layer, and a second conductive typelayer are sequentially formed on the crystal layer so as to extendwithin planes parallel to the tilt crystal plane, tilted from theprincipal plane of the substrate, of the crystal layer.

The first conductive type layer is a p-type or n-type cladding layer,and the second conductive type layer is an n-type or p-type claddinglayer.

For example, in the case of forming the crystal layer having the S-planeby using a gallium nitride based compound semiconductor, the n-typecladding layer made from a silicon-doped gallium nitride based compoundsemiconductor may be formed on the S-plane of the crystal layer, anactive layer made from InGaN be formed on the n-type cladding layer, andthe p-type cladding layer made from magnesium-doped gallium nitridebased compound semiconductor be formed on the active layer. Thesemiconductor light emitting device thus produced has a so-calleddouble-hetero structure.

The active layer may have a structure that an InGaN layer be sandwichedbetween AlGaN layers. Also, the active layer may be of a single bulklayer structure, or a quantum well structure such as a single quantumwell (SQW) structure, a double quantum well (DQW) structure, or multiplequantum well (MQW) structure. The quantum well structure uses a barrierlayer for separation of quantum wells, if necessary.

The provision of the InGaN layer as the active layer is particularlyadvantageous in terms of easy fabrication of the light emitting deviceand improvement of light emission characteristics of the light emittingdevice. The InGaN layer grown on the S-plane is further advantageous inthat since the S-plane has a structure that nitrogen atoms are lessreleasable, the crystallization of InGaN on the S-plane is particularlyeasy and the crystallinity of InGaN formed on the S-plane is desirable.

Additionally, a nitride semiconductor has a property to become n-typeconductive even in the non-doped state because of nitrogen holesoccurring in crystal; however, the nitride semiconductor may beconverted into an n-type semiconductor with a desirable concentration ofcarriers by doping an ordinary donor impurity such as Si, Ge, or Seduring crystal growth of the nitride semiconductor.

A nitride semiconductor can be converted into a p-type semiconductor bydoping an acceptor impurity such as Mg, Zn, C, Be, Ca, or Ba in crystalof the nitride semiconductor. In this case, to obtain a p-layer with ahigh carrier density, after being doped with the acceptor impurity, thenitride semiconductor may be activated, for example, by an annealingtreatment performed at about 400° C. or more in an inert gas atmospheresuch as a nitrogen or argon atmosphere. The activation of the nitridesemiconductor may be performed by irradiating the nitride semiconductorwith electron beams, microwaves, or light.

The first conductive type layer, the active layer, and the secondconductive type layer can be easily formed on the crystal layer so as toextend within planes parallel to the tilt crystal plane, tilted from theprincipal plane of the substrate, of the crystal layer by continuouslyforming these layers on the tilt crystal plane of the crystal layer bycrystal growth. If the crystal layer has an approximately hexagonalpyramid or approximately hexagonal truncated pyramid shape whose tiltcrystal plane is the S-plane, the light emission region composed of thefirst conductive type layer, the active layer, and the second conductivetype layer can be wholly or partially formed on the S-plane. If thecrystal layer has an approximately hexagonal truncated pyramid shape,the first conductive type layer, the active layer, and the secondconductive type can be formed even on an upper plane, parallel to theprincipal plane of the substrate, of the truncated pyramid shape.

In the case of forming the light emission region on the plane parallelto the principal plane of the substrate, light emitted from the lightemission region is decayed by multiple reflection, whereas in the caseof forming the light emission region on the tilt S-plane tilted from theprincipal plane of the substrate, light emitted from the light emissionregion can be emerged to the outside of the light emitting semiconductordevice without occurrence of multiple reflection.

The first conductive type layer functioning as the cladding layer can bemade from the same material as that of the crystal layer so as to havethe same conductive type as that of the crystal layer. To be morespecific, the first conductive type layer can be formed by continuing,after the crystal layer having the S-plane is formed, the crystal growthwhile continuously adjusting the concentration of the source gas.Alternatively, the first conductive type layer may be configured as partof the crystal layer having the S-plane. In addition, to improve thelight emergence efficiency, the first conductive type layer may beformed on the plane not parallel to the principal plane of thesubstrate.

According to the semiconductor light emitting device in this embodiment,the luminous efficiency can be increased by making use of a desirablecrystallinity of the tilt crystal plane, tilted from the principalplane, of the crystal layer. In particular, by injecting a current onlyinto the S-plane having a desirable crystallinity, it is possible toenhance the luminous efficiency. This is because the InGaN active layercan be desirably formed on the S-plane having a desirable crystallinity.In addition, the actual area of the active layer extending within aplane being substantially parallel to the S-plane is larger than thearea, projected on the principal plane of the substrate or theunderlying growth layer, of the active layer. The enlarged area of theactive layer makes it possible to increase the area of the lightemission region of the device and thereby reduce the density of acurrent injected in the light emission region, and to reduce thesaturated luminance and thereby increase the luminous efficiency.

With respect to the semiconductor light emitting device including thehexagonal pyramid shaped crystal layer having the tilt S-plane, thestepped state of the surface of a portion near the top of the S-planebecomes poor, so that the luminous efficiency at the top portion of thedevice is degraded.

To be more specific, when the S-plane section on one side of thehexagonal pyramid shape is divided into four regions (top region, leftregion, right region, and bottom region) with respect to a nearlycentral portion of the S-plane section, the stepped state is most wavyin the top region, whereby abnormal crystal growth is liable to occur inthe top region. On the contrary, in each of the left and right regions,since steps extend nearly in straight line and are closely collected,the crystal growth state becomes desirable. In the bottom region,although steps are slightly wavy, crystal growth is not so abnormal asobserved in the top region.

In the semiconductor light emitting device of the present invention, itis thus recommended that the injection of a current in the active layerbe controlled such that the current density in the top region be smallerthan that in each of the other regions. To make the current density inthe top region small, an electrode may be formed not in the top regionbut in the side region, or a current blocking area be formed in the topregion before an electrode is formed in the top region.

An electrode is formed on each of the crystal layer and the secondconductive type layer. To reduce the contact resistance, a contact layermay be formed and then the electrode be formed thereon. In the case offorming these electrodes by vapor deposition, if the p-electrode and then-electrode adhere on both the crystal layer and the crystal seed layerformed under the mask layer, there occurs short-circuit therebetween. Tocope with such an inconvenience, each of the electrodes must beaccurately formed by vapor deposition.

An image display unit or an illumination unit can be fabricated byarraying a plurality of the semiconductor light emitting devicesaccording to the present invention. In this case, according to thesemiconductor light emitting device of the present invention, theelectrode area can be suppressed by making use of the S-plane, andaccordingly, by preparing the semiconductor light emitting devices ofthree primary colors and arraying them in a scannable manner, an imagedisplay unit with a reduced electrode area can be realized.

The shape of the semiconductor light emitting device of the presentinvention can be variously changed as described below with reference toexamples shown in FIGS. 6 to 10.

FIG. 6 shows a first example in which each stripe-shaped crystal growthlayer is formed on a growth substrate. As shown in the figure, anunderlying growth layer 21 is formed on a growth substrate 20, a masklayer 22 having window regions is formed on the underlying growth layer21, and stripe-shaped crystal growth layers 24 are formed by selectivecrystal growth from the window regions. In the stripe-shaped crystalgrowth layer 24, both side surfaces 26 are each taken as the S-plane. Anactive layer 25 is formed on each crystal growth layer 24 in such amanner as to extend on both the tilt side surfaces 26 and an uppersurface of the crystal growth layer 24. The area of the active area 25is larger than the area, projected on the horizontal plane, of thecrystal growth layer 24. As a result, it is possible to effectivelyrelieve the saturated luminance and hence to improve the reliability ofthe device.

FIG. 7 shows a second example in which each rectangular trapezoidalcrystal growth layer is formed on a growth substrate. As shown in thefigure, an underlying growth layer 31 is formed on a growth substrate30, a mask layer 32 having window regions is formed on the underlyinggrowth layer 31, and stripe-shaped rectangular trapezoidal crystalgrowth layers 33 are formed by selective growth from the window regions.In the rectangular trapezoidal crystal growth layer 33, both sidesurfaces 33S are each taken as the S-plane, both longitudinal endsurfaces 34 are each taken as the (11-22) plane, and an upper surface33C is taken as the C-plane being the same as that of the principalplane of the growth substrate 30. While not shown, an active layer isformed on each crystal growth layer 33 in such a manner as to extend onthe tilted side surfaces 33S, the end surfaces 34, and the upper surface33C. The area of the active layer is larger than the area, projected onthe horizontal plane, of the crystal growth layer 33. As a result, it ispossible to effectively relieve the saturated luminance and hence toimprove the reliability of the device.

FIG. 8 shows a third example in which each square truncated pyramidshaped crystal growth layer is formed on a growth substrate. As shown inthe figure, an underlying growth layer 41 is formed on a growthsubstrate 40, a mask layer 42 having window regions is formed on theunderlying growth layer 41, and square truncated pyramid shaped crystalgrowth layers 43 are formed by selective crystal growth from the windowregions in such a manner as to be arrayed in a matrix pattern. In thesquare truncated pyramid shaped crystal growth layer 43, a pair ofopposed tilt side surfaces 43S are each taken as the S-plane, anotherpair of opposed tilt side surfaces 44 are each taken as the (11-22)plane, and an upper surface 43C is taken as the C-plane being the sameas that of the principal plane of the growth substrate 40. While notshown, an active layer is formed on each crystal growth layer 43 in sucha manner as to extend on the tilted side surfaces 43S and 44, and theupper surface 43C. The area of the active layer is larger than the area,projected to horizontal plane, of the crystal growth layer 43. As aresult, it is possible to effectively relieve the saturated luminanceand hence to improve the reliability of the device.

FIG. 9 shows a fourth example in which each hexagonal pyramid shapedcrystal growth layer is formed on a growth substrate. As shown in thefigure, an underlying growth layer 51 is formed on a growth substrate50, a mask layer 52 having window regions is formed on the underlyinggrowth layer 51, and hexagonal pyramid shaped crystal growth layers 53are formed by selective crystal growth from the window regions in such amanner as to be arrayed in a matrix pattern. In the hexagonal pyramidshaped crystal growth layer 53, side surfaces are each taken as theS-plane. While not shown, an active layer is formed on each crystalgrowth layer 53 in such a manner as to extend on the tilt S-planes. Thearea of the active layer is larger than the area, projected tohorizontal plane, of the crystal growth layer 53. As a result, it ispossible to effectively relieve the saturated luminance and hence toimprove the reliability of the device.

FIG. 10 shows a fifth example in which each hexagonal truncated pyramidshaped crystal growth layer is formed on a growth substrate. As shown inthe figure, an underlying growth layer 61 is formed on a growthsubstrate 60, a mask layer 62 having window regions is formed on theunderlying growth layer 61, and hexagonal truncated pyramid shapedcrystal growth layers 63 are formed by selective crystal growth from thewindow regions in such a manner as to be arrayed in a matrix pattern. Inthe hexagonal truncated crystal growth layer 63, side surfaces 63S areeach taken as the S-plane, and an upper surface 63C is taken as theC-plane being the same as that of the principal plane of the substrate.In addition, a small-height portion having the M-plane, that is, the(1-100) plane is also formed on the bottom surface side of the hexagonaltruncated pyramid shaped crystal growth layer 63. While not shown, anactive layer is formed on each crystal growth layer in such a manner asto extend on the tilt S-planes and the C-plane. The area of the activelayer is larger than the area, projected to the horizontal plane, of thecrystal growth layer 63. As a result, it is possible to effectivelyrelieve the saturated luminance and hence to improve the reliability ofthe device.

As described above, according to the semiconductor light emitting deviceof the present invention, it is possible to keep the crystal growth rateof an active layer nearly at a constant value, and hence to eliminate avariation in thickness of the active layer and thereby solve the problemassociated with the degradation of characteristics of the device.Further, it is possible to make the thickness of one of multiple quantumwells nearly equal to that of another of the multiple quantum wells, andhence to enhance light emission characteristics of the device, forexample, reduce a half-value width of an emission wavelength peak.

While the preferred embodiments of the present invention have beendescribed using the specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit and scope of the followingclaims.

1. A method of fabricating a semiconductor light emitting deviceincluding a cladding layer, a guide layer, and an active layer, each ofwhich is sequentially formed by selective growth, said method comprisingthe steps of: previously growing crystal in an active layer non-growthregion; and growing said active layer in an active layer selectivegrowth region, wherein said cladding layer, said guide layer, and saidactive layer are sequentially grown on a tilt growth plane of a nitridegrown on a sapphire substrate through a mask having an opening.
 2. Amethod of fabricating a semiconductor light emitting device according toclaim 1, wherein said active layer has a multi-layer structure.
 3. Amethod of fabricating a semiconductor light emitting device according toclaim 1, wherein said active layer has multiple quantum wells.
 4. Amethod of fabricating a semiconductor light emitting device according toclaim 1, wherein said active layer is grown by an MOCVD process.
 5. Amethod of fabricating a semiconductor light emitting device including acladding layer, a guide layer, and an active layer, each of which issequentially formed by selective growth, said method comprising thesteps of: previously growing crystal in an active layer non-growthregion; and growing said active layer in an active layer selectivegrowth region wherein said active layer is grown by an MOCVD process,and wherein said crystal is grown on said non-growth region by changinga carrier gas into nitrogen gas in said MOCVD process.
 6. A method offabricating a semiconductor light emitting device including a claddinglayer, a guide layer, and an active layer, each of which is sequentiallyformed by selective growth, said method comprising the steps of:previously growing crystal in an active layer non-growth region; andgrowing said active layer in an active layer selective growth regionwherein said active layer is grown by an MOCVD process, and wherein saidcrystal is grown in said non-growth region by lowering a growthtemperature in said MOCVD process.